The invention can be applied to electrical distribution networks such as for example those on board transport means, in particular in the aeronautical, automotive or rail fields. These networks make it possible to supply power from an AC voltage source comprising at least one phase to various devices requiring DC voltages.
In the electrical networks, the power supply must address multiple sub-assemblies or consumers connected to said network. A problem arises when one or more of these sub-assemblies is short-circuited. Specifically, unlike power supplies serving only one electrical device, cutting the power supply in order to disconnect it from the network, for example by tripping a protective device or melting a fuse, risks cutting the supply of power to the other consumers connected to the electrical network. If a consumer becomes faulty, the power supply must not be interrupted and must be capable of activating a safety measure configured to isolate the faulty consumer from the network.
One problem with power converters implemented using power factor correction to regulate their output current is due to the behavior of this power structure when its output is short-circuited or when the amplitude of the output voltage is very low. Specifically, the transfer function of such a structure comprises a static gain that is proportional to the inverse of the output voltage of the power structure.
With reference to FIG. 1, it is recalled that when an AC-to-DC power conversion circuit of power factor corrector (PFC) type is connected to an AC network, it does not disrupt, or hardly disrupts, the latter. To achieve this, the PFC circuit must have a power factor that is as close as possible to unity and have very little in the way of input current harmonics. Its input current must be sinusoidal and in phase with the voltage at its input. Viewed from its input, the converter must be as close as possible to a resistive load. To this end, the power structure comprises a primary current loop. The input current of the power structure is slaved to a setpoint signal that is proportional to the input voltage of said power structure and therefore takes the sinusoidal form thereof. Controlling the amplitude of this setpoint signal allows the amplitude of the current absorbed by the power structure to be controlled. In order to maintain a gain that is independent of the input voltage between the control signal B and the power absorbed by the power structure, the input current setpoint signal, consigne_Iin, is usually determined as follows:consigne_Iin=A×B/C2                 Where: A is a signal proportional to the instantaneous input voltage, it therefore has the sinusoidal form thereof;                    B is a control signal allowing the power absorbed by the power structure to be controlled;            C is a signal proportional to the RMS value of the input voltage of the power structure.                        
By way of illustration, FIG. 2 shows an exemplary embodiment of a power structure of flyback type known from the prior art. Let Uin(t) be the input voltage. This voltage may be written in the form:Uin(t)=Ueff×√2×sin(ω×t)                where Ueff represents the RMS value of the input voltage.        
Following the preceding descriptions, it is possible to write:A=K1×Ueff×√2×sin(ω×t)C=K2×Ueff                 Where K1 and K2 are two constants        
If it is assumed that the PFC is operating correctly, the input current I follows its setpoint and hence:I=consigne_Iin=K1×Ueff×√2×sin(ω×t)×B/K22×U2eff 
The RMS value of the input current can therefore be extracted:Ieff=K1×B/(K22×Ueff)
Since the current and the voltage are in phase, the input power Pin may be written as:Pin=Ieff×Ueff=(K1/K22)×B 
It is observed that the input power, and hence the delivered power (efficiency apart), varies only with the term B. Since the constants K1 and K2 are defined by the measurement circuit and hence set in the design phase, the power structure, including the primary current feedback control loop, therefore delivers an output power that is proportional to B.
In order to precisely control the output current, it is feedback-controlled. FIG. 3 schematically shows an exemplary secondary current loop. In nominal operation, the output current setpoint is defined by the corrector feedback-controlling the output voltage. In the event of short-circuit operation, a fixed value corresponding to the desired short-circuit current is applied.
Since the corrector feedback-controlling the output current directly controls the “B” input of the power structure including the primary current loop, this results in the output power being controlled. The relationship between output power and output current produces a gain that is proportional to the inverse of the output voltage (1/Vs) in the secondary current loop. For a given output voltage Vs, the sizing of the corrector of the output current loop does not present any problems. However, the stability criteria of the loops (gain and phase margins) cannot be guaranteed as the voltage drops. Since the gain of the open-loop transfer function (FBTO) increases while the phase remains unchanged, the drop in the output voltage Vs leads to an increase in the passband and a decrease in the phase margin, until the system becomes unstable.
By way of example, FIGS. 4 to 6 show Bode plots (magnitude and phase) of the open-loop transfer function of one and the same power structure for a case of a nominal voltage of 42 V, in the case of the voltage having fallen to 20 V and in the case of the voltage having fallen to 7 V. In the case of the output voltage being at its nominal value, the structure has a passband of 220 Hz, a gain margin of 15.5 dB and a phase margin of 42°. When the output voltage drops to 20 V, the passband increases to the detriment of the stability margins. The passband goes to 340 Hz while the gain margin and the phase margin fall to 9 dB and 22°, respectively. In the latter example, instability is reached, the phase and gain margins are zero.
One problem therefore arises when the output of the power structure is short-circuited or when the amplitude of the output voltage is very low. In this case the current loop becomes unstable and instability of the output current loop results in a large oscillation of the output current, hence a loss of control of said output current. This may result in the converter overheating, or even being destroyed.
Power converters in which stability is ensured by the circuit at the primary of the power structure are known, but these present the problem of controlling the output current from the primary circuit to the secondary of the power structure.